1. Field of the Invention
The present invention relates to a camera having a flash emission device which emits flashing light and used to take a picture of a subject.
2. Description of the Related Art
Many cameras have a flash emission device which emits flashing light toward a subject. From such cameras, flashing light is emitted in synchronization with a picture-taking operation. When this flash photography is performed, the aperture to be set when a picture is taken is determined on the basis of the luminance of field, film speed information about a loaded film, etc., and the amount of flashing light is adjusted according to the aperture. Thus, the amount of light applied to the film is optimized by adjusting the amount of flashing light.
Methods of adjusting the amount of light include a method of adjusting the amount of light through the time period during which flashing light is emitted.
In a camera having a flash emission device using this method, however, a phenomenon occurs in which if the temperature of a certain portion decreases after adjustment of the amount of flashing light through the control of emission time, the amount of emitted light becomes smaller than a target amount, and in which if the temperature rises, the amount of emitted light becomes larger than the target amount. When this phenomenon occurs, there is a possibility of failure to obtain the desired exposure.
This is because the internal resistance of a capacitor provided in the flash emission device and used for emitting flashing light, etc., change when the temperature changes.
A camera has therefore been proposed in which temperature is detected and flashing light is emitted for an emission time period selected by considering the detected temperature information (see, for example, Patent Document 1).
In the proposed camera, a reference table in which predetermined emission time periods are set with respect to predetermined temperature ranges and amounts of flashing light determined according to distance information and film speed information. In this camera, an emission time period determined by considering temperature information as well as other kinds of information is obtained from the reference table and, therefore, the target amount of flashing light can be emitted with stability.
Patent Document 2 also proposes a related technique, which discloses to a focal-point-detecting preliminary lighting device for emitting auxiliary flashing light when detecting a distance to a subject. Specifically, this document discloses a technique for variably setting the amount of emitted light at the time of measurement by detecting a charging voltage during emission of flashing light for emitting auxiliary light toward the subject.
Also, there is a possibility of stoppage of flashing due to a reduction in voltage across a main capacitor when a focal plane shutter is made to travel. A camera is known in which such stoppage of flashing is prevented in such a manner that the luminous intensity of emitted flashing light is made constant over the exposure time and the amount of emitted light is uniformly applied to the entire film, although the amount of light becomes slightly short (see Patent Document 3).
[Patent Document 1]
Japanese Patent Laid-Open No. 2002-72311
[Patent Document 2]
Japanese Patent Laid-Open No. 7-92371
[Patent Document 3]
Japanese Patent Laid-Open No. 7-120813
In the camera proposed in Patent Document 1, emission of a target amount of flashing light may be stabilized with high accuracy in such a manner that the above-mentioned temperature ranges are further divided into narrower temperature ranges and different flashing times are set in a one-to-one relationship with the narrower temperature ranges. However, such subdividing of the temperature ranges is limited due to memory capacity.
Some cameras have a red-eye prevention mode which is provided for prevention or reduction of red-eye flash photography and in which a preliminary flash is generated immediately before taking a picture and a main flash is thereafter generated in synchronization with a picture-taking operation. When flash photography is performed by setting the red-eye prevention mode in this type of camera, the voltage across a main capacitor is reduced by generation of a preliminary flash. Therefore it is difficult to set a suitable amount of light for a main flash.
Patent Document 1 shown above discloses a technique for correcting the emission time according to the temperature inside the camera. However, this technique is irrelevant to correction of variation in the amount of light due to a reduction in the voltage across the main capacitor.
According to Patent Document 2 shown above, the amount of emitted auxiliary light is changed by detecting the voltage across a capacitor in a flash emission means. However, this is a technique for roughly changing the amount of emitted light based on comparison of the voltage across the capacitor with a plurality of threshold values. This technique cannot be used for flash photography.
According to Patent Document 3, the voltage across a main capacitor in a flash is also detected for correction, and the luminous intensity of emitted light is controlled so that emission of light from the flash is maintained during the emission time determined by the curtain speed and the shutter speed of a focal plane shutter, while a slight deficiency of the amount of exposure is allowed. A suitable amount of light during main light emission cannot be obtained by using this technique.
In view of the above-described circumstances, an object of the present invention is to provide a camera devised in such a manner that flash emission time periods are set in correspondence with small differences in temperature to maintain with high accuracy a target amount of flashing light to be emitted.
Another object of the present invention is to provide a camera capable of optimizing the amount of emitted flashing light in a main flash even in a case where preliminary light emission and main light emission are continuously performed.
To achieve the above-described objects, according to a first aspect of the present invention, there is provided a camera which takes a picture of a subject, the camera including a flash emission device which emits flashing light in synchronization with a picture-taking operation, and which controls the amount of emitted light by selecting an emission time period, the flash emission device having a temperature sensor which detects temperature, a storage section which stores a computation expression by which an emission time period for obtaining a predetermined amount of emitted light is obtained by using temperature as a variable, and an emission control section which obtains an emission time period from the temperature obtained from the temperature sensor on the basis of the computation expression, and which controls the emission time period so that light is emitted only for the obtained emission time period.
In the camera according to the first aspect of the present invention, the target amount of emitted light is ensured by making the flash emission device emit light for the emission time period obtained by the computation expression using the temperature as a variable. That is, in the camera according to the first aspect of the present invention, constants of the computation expression can be stored in a memory. That is, in the camera according to the first aspect of the present invention, if constants of the computation expression are stored in a memory, flash emission time periods corresponding to small differences in temperature can be set by using the detected temperature as a variable. Therefore the camera according to the first aspect of the present invention is capable of maintaining the target amount of emitted flashing light with improved accuracy in comparison with the conventional camera using the same memory capacity.
Preferably, the computation expression is represented by an expression including a linear expression in each of temperature ranges and expressed by a polygonal line as a whole.
This computation expression is capable of finely setting the emission time period with respect to variation in temperature without considerably sacrificing the accuracy in comparison with an expression of a higher order.
To solve the above-described problem, according to a second aspect of the present invention, there is provided a camera which takes a picture of a subject, the camera including a flash emission device which performs preliminary light emission before a picture-taking operation, and which performs main light emission in synchronization with the picture-taking operation, the flash emission device having a main capacitor in which charge for emission of flashing light is accumulated, a light emitting section which emits flashing light by receiving electric power from the main capacitor, a voltage monitor which detects the voltage across the main capacitor before the main light emission, an emission time computation section which obtains, on the basis of the detected voltage obtained by the voltage monitor, an emission time period such that the same amount of light as the amount of light when flashing light is emitted for a predetermined reference emission time period when the voltage across the main capacitor is equal to a predetermined reference voltage can be obtained, and an emission control section which controls light emission so that flashing light is emitted only for the emission time period obtained by the emission time computation section.
In the camera according to the second aspect of the present invention, when main light emission is performed in synchronization with a picture-taking operation after preliminary light emission has been performed before the picture-taking operation, an emission time period such that the same amount of light as the amount of light when flashing light is emitted for a predetermined reference emission time period when the voltage across the main capacitor is equal to a predetermined reference voltage can be obtained is computed by the emission time computation section on the basis of the detected voltage obtained by the voltage monitor, and the emission time period is adjusted. Emission of flashing light is controlled by the emission control section so that flashing light is emitted only for the adjusted emission time period. Thus, the emission time period for main light emission is adjusted according to the voltage across the main capacitor when preliminary light emission and main light emission is continuously performed, thereby optimizing the amount of light at the time of main light emission.
Preferably, in camera according to the second aspect of the present invention, the aperture at the time of main light emission is variable and the reference emission time period is a time period according to the aperture at the time of main light emission.
xe2x80x9cThe time of main light emissionxe2x80x9d refers to a time at which main light emission is actually performed, and actual examples are as follows. Some cameras have, for example, a programmable shutter having the function of setting an aperture and being gradually opened with the passage of time to reach the maximum opening. With respect to a type of camera in which main light emission is performed when a shutter gradually opened and has an aperture diameter according to the subject distance and the subject luminance, xe2x80x9cthe time of main light emissionxe2x80x9d corresponds to the time at which this aperture is reached.
A type of camera in which main light emission is performed always when the maximum opening is reached may be such that, for example, a zoom lens is provided and the aperture diameter at the maximum opening varies depending on the zooming position of the zoom lens. In such a case, xe2x80x9cthe aperture at the time of main light emissionxe2x80x9d is the maximum opening according to the zooming position in a picture-taking operation using the main light emission. The present invention can also be applied to a type of camera in which an aperture and a shutter are provided separately from each other, and xe2x80x9cthe aperture at the time of main light emissionxe2x80x9d refers to the aperture diameter at which main light emission is performed in a picture-taking operation.
Thus, in a camera using a variable-aperture programmable shutter, the emission time period can be correctly adjusted to a time period according to the aperture at the time of main light emission.
Preferably, the camera has a temperature sensor which detects temperature, and the reference emission time period is a time period according to the temperature detected by the temperature sensor.
In such a case, flashing light can be emitted for an emission time period according to the temperature of the camera, and the amount of light can be optimized even when the temperature of the camera changes.
Further, preferably, the camera has a distance sensor which detects a subject distance, and the reference emission time period is a time period according to the subject distance detected by the distance sensor.
In such a case, the emission time period can be accurately adjusted to a time period according to the subject distance.
Preferably, the above-described emission time computation section stores a computation expression by which the time period from the emission start instruction to the emission stop instruction is obtained. Also, if the reference emission time period is Tf; the reference voltage is Vf; the detected voltage is V; a constant corresponding to a time delay from a moment at which an emission start instruction is issued to a moment at which light emission is started is t0; and the emission time period is T, the emission time computation section obtains the emission time period T by
T=(Tfxe2x88x92t0)xc3x97(Vf/V)+t0xe2x80x83xe2x80x83(1) 
In this manner, the emission time period can be computed by considering the detected voltage V and the constant t0 corresponding to a time delay from a moment at which an emission start instruction is issued to a moment at which light emission is started.
Preferably, when the reference emission time period Tf is longer than a predetermined time period T1 of 30 xcexcsec or longer, the emission time computation section obtains the emission time period T by
T=Tfxc3x97(Vf/V)xe2x80x83xe2x80x83(2) 
instead of the equation (1).
When the emission time period T is longer than the predetermined time period T1, 30 xcexcsec in this case, the emission time period obtained by the equation (2) is substantially the same as that obtained by the equation (1). Therefore the emission time period is computed by the equation (2). The computation in the emission time computation section is thereby simplified to reduce the load on the emission time computation section.
As described above, in the camera according to the first aspect of the present invention, flash emission times can be finely set with respect to small variations in temperature and a target amount of emitted flashing light can be maintained with higher accuracy in comparison with the conventional cameras.
In the camera according to the second aspect of the present invention, the amount of emitted flashing light in a main light emission can be optimized even in a case where preliminary light emission and main light emissions are performed continuously.